The use of a one time pad protocol based on a shared secret key is of special interest in cryptography, since it can provide unbreakable encryption. However, messages encrypted with this protocol can be decrypted by anyone in possession of the key, so the protocol vulnerability depends on the security of key distribution. It is helpful to formulate the key distribution problem in the following standard manner. Alice and Bob communicate with each other via a channel. An eavesdropper Eve has full access to the channel. Eve can receive, tap and/or intercept signals from Alice and Bob, and can also send signals to Alice and Bob. Signals between Alice and Bob can include key information and/or encrypted messages.
Quantum key distribution (QKD) is of special significance, since it can provide provably secure key distribution over a compromised channel. The main idea of quantum key distribution is that Eve's actions in monitoring signals from Alice and Bob cannot be performed without modifying these signals. In other words, an “undetectable tap” does not physically exist. This perturbation of the signals is a quantum-mechanical effect, and key distribution protocols based on various kinds of quantum states have been proposed. Examples include the use of entangled states, non-orthogonal states, orthogonal states (U.S. Pat. No. 6,188,768), and states from single-photon sources (US 2005/0094818). The various QKD methods differ significantly in terms of their performance (as measured by secure key distribution rate) and their technical requirements (which affect cost). In fact, US 2005/0152540 proposes a hybrid key distribution scheme to use a short key (distributed by a slow QKD method) to provide fast and secure key distribution.
For simplicity, it is preferable to employ a QKD method that can directly provide fast and secure key distribution without excessive cost (e.g., preparation of exotic quantum states). A promising approach is differential phase shift (DPS) QKD, which was proposed in connection with a single photon source by Inoue et al., Phys. Rev. Lett., 89(3), 037902, 2002. DPS QKD was extended to pulses from a coherent source by Inoue et al. in Phys. Rev. A, 68, 022317, 2003. Although these references indicate that DPS QKD can outperform conventional QKD protocols such as BB84 and B92, a full security analysis of DPS QKD is not provided in this work. Without such an analysis, it is not clear how to maximize (or nearly maximize) the secure key distribution rate for DPS QKD given various system parameters (e.g., transmission loss, detector efficiency, etc.).
Accordingly, it would be an advance in the art to provide DPS QKD that can be tailored to maximize (or nearly maximize) the secure key distribution rate.